Martian volcanoes are not known for being hard to spot. The behemoth Olympus Mons, for example, claims the mantle of the largest known volcano in the Solar System. A new paper published in Nature, however, suggests that some volcanoes on the red planet have remained incognito by blending in with the impact craters that dot Mars’ surface. The reason for this mistaken identity is that these volcanoes undergo explosive, Yellowstone-style eruptions, leaving a crater-like caldera behind.

Olympus Mons and its kin are shield volcanoes—broad domes built up by lava flows issuing from a central vent. Volcanism has taken a number of other forms throughout the planet’s history, though, before activity dropped off as Mars cooled. Erosion and burial beneath younger lava flows have made it harder to identify some volcanic features—especially the older ones.

One thing that isn’t hard to find is an impact crater. These scars left by collisions with other bodies are numerous and have a set of distinctive features—there's more to a crater than a hole in the ground. You can usually find a raised area like a bulls-eye in the center of the circular depressions. The rim around the edge is raised above its surroundings. The material that was rudely excavated by the impact is launched radially outward, forming what’s known as an “ejecta blanket” where it settles to the ground.

In a pock-marked region named Arabia Terra, researchers Joseph Michalski and Jacob Bleacher have spotted a few depressions that they think are not like the others. They lack those characteristic features of impact craters, including the neatly circular shape. Instead, they look strikingly like volcanic calderas created by massive eruptions.

Calderas form when the contents of large magma chambers are explosively vomited into the air. With nothing left in the chamber to support the overlying rock, the surface collapses in. If you expected all volcanoes to be impressive, Olympus Mons style peaks, you would miss Yellowstone Caldera, the source of an eruption that dumped ash over the majority of the United States just 640,000 years ago. Michalski and Bleacher think they’ve been looking at calderas on Mars associated with eruptions at least that large.

Their best example is a feature called the Eden patera (pictured above). The nearly 2-kilometer-deep depression lies at the center of a shallow bowl in the surface. Careful inspection reveals signs of the type of faults that form as calderas collapse in on themselves. The convoluted shape of the depression, difficult to explain as an impact, could be the result of several overlapping calderas from separate eruptions. In one portion, the researchers even noticed terraces that resemble the “bathtub rings” left by drained lava lakes.

If these really are calderas, it could add a lot to our knowledge of Mars’ volcanic history. For one thing, they could be the source of some ancient layered rocks on Mars that researchers have puzzled over for some time.

It would also point to a different style of volcanism in Mars’ youth, when its crust was much thinner. If they were widespread, these massive eruptions could have wreaked a lot of havoc on the climate system. Since we want to know how Mars lost its water, its climate history is something we’re keenly interested in learning more about.

Maybe that site shown in the video should be another destination for the next Curiosity. If that really is a Crater Lake-like caldera and it happened early in Mars' history, then it's likely that it filled with water just like calderas on Earth. There will be sediment to prove it.

I wouldn't think that the Mars' core cooled off so much faster than Earth's core.

Because Mars is so much smaller than the Earth, it has, in fact, cooled enough that the surface appears to be geologically "dead". That doesn't mean the core isn't hot, but there's nothing going on anywhere near the surface.

I wouldn't think that the Mars' core cooled off so much faster than Earth's core.

Mars's core cooled off enough faster that it's magnetic field died out after a few hundred million years. Remember that while Mars has 53% of Earth's radius and 38% of its surface gravity it only has 10% of the mass so it is a much smaller body and would be expected to cool significantly faster.

That said, we don't currently have enough information to create detailed models of the Martian interior. Nasa's InSight mission is scheduled for a 2016 launch and is designed to gather seismic and detailed rotation data on the planet to answer the questions.

Interesting. Both Mars and Venus are magnetically dead bodies, Venus lost its magnetic field, when its rotation on its axis was severely affected from a hypothized impact with a planetesimal, yet the planet is still geologically active with thick atmosphere.

Mars is on other hand is still rotating, yet it is geologically dead with very thin atmosphere that is 1% of density of earth atmosphere, and add that it is noticeably least densest rocky planet.

Could mars simply spewed out most of its contents needed to keep its magnetic field in form of massive eruptions

If only we evolved a few billions years earlier we might have seen a much different solar system and might possibly have had up to two more worlds to visit that possibly had much friendlier environments for us (mars and Venus). C'est la vie.

It would be interesting to see if there were any sign of erosion in the crater.

I was thinking along the same lines. But I was wondering if weathering could eventually wear down the raised rim of a crater and fill in the center making the center cone (are there names for the crater features?) less prominent, or in time invisible.

Mars may not have earths atmosphere, but there is enough for windborn erosion.

And if there were any houses the dust devils probably took them to oz.

If only we evolved a few billions years earlier we might have seen a much different solar system and might possibly have had up to two more worlds to visit that possibly had much friendlier environments for us (mars and Venus). C'est la vie.

Now, if we want to scatter eggs out of this basket, we must engineer our own pleasantness or figure out how to jump interstellar distances to find endemic pleasantness.

If only we evolved a few billions years earlier we might have seen a much different solar system and might possibly have had up to two more worlds to visit that possibly had much friendlier environments for us (mars and Venus). C'est la vie.

If only we evolved a few billions years earlier we might have seen a much different solar system and might possibly have had up to two more worlds to visit that possibly had much friendlier environments for us (mars and Venus). C'est la vie.

We can still visit mars.

Venus, not so much despite her being such a hot babe.

Yeah I just imagine what it'd be like to visit either Mars or Venus and enjoy a climate much closer to present day Earth.

Then again Earth was a much different planet that far back so I guess we'd be screwed one way or another.

The more old volcanoes the merrier: there's a recent theory that predicts that Mars habitability was ~ 3.8 - 3.6 Ga bp, coeval with Gale crater habitability as established by Curiosity, from river networks. Presumably such a late and short surface habitability of the atmosphere was fed by volatiles from Tharsis, and perhaps these.

Annoyingly the Nature paper is paywalled, and Ars won't tell us the relative dating. [/pouts]

Martian meteorites tells us it was geologically active (magma chambers) ~ 100 Ma ago. That is 0.2 % of its entire lifetime. It would take a terrible finetuning to have it die after 99.8 % of the maximally observable period.

And indeed, there is at least one paper that claims that some boulder movements can only be predicted by marsshakes within the last 100 000 years or so.

We need the insight from upcoming InSIGHT to tell. It will look at the core and its current state (liquid or frozen).

Interesting. Both Mars and Venus are magnetically dead bodies, Venus lost its magnetic field, when its rotation on its axis was severely affected from a hypothized impact with a planetesimal, yet the planet is still geologically active with thick atmosphere.

Mars is on other hand is still rotating, yet it is geologically dead with very thin atmosphere that is 1% of density of earth atmosphere, and add that it is noticeably least densest rocky planet.

Could mars simply spewed out most of its contents needed to keep its magnetic field in form of massive eruptions

I don't think so. Volcano's release molten rock from the upper mantle; the magnetic field is generated in the core. The two regions are separated by the bulk of the planets radius.

That said, we don't currently have enough information to create detailed models of the Martian interior. Nasa's InSight mission is scheduled for a 2016 launch and is designed to gather seismic and detailed rotation data on the planet to answer the raise more questions.

Interesting. Both Mars and Venus are magnetically dead bodies, Venus lost its magnetic field, when its rotation on its axis was severely affected from a hypothized impact with a planetesimal, yet the planet is still geologically active with thick atmosphere.

Mars is on other hand is still rotating, yet it is geologically dead with very thin atmosphere that is 1% of density of earth atmosphere, and add that it is noticeably least densest rocky planet.

Could mars simply spewed out most of its contents needed to keep its magnetic field in form of massive eruptions

I don't think so. Volcano's release molten rock from the upper mantle; the magnetic field is generated in the core. The two regions are separated by the bulk of the planets radius.

You definitely right about magnetic field being generated in the core, but what about the iron content in the planet core itself? I read not long ago about the inner planets densitties, Mars has lowest of about 3.940 gr/cm 3, the rest are above 5 gram/cm 3, the reason for this is because Mars has less dense materials than other three, and has more percentage of silicate rock in its composition than the rest of inner planet. As far as I know, iron is the element from which the core generates its magnetic field for rocky planets.